Electrical insulating material



Patented Feb. 21, 1939 UNITED STATES ELECTRICAL INSUIATING MATERIAL Jolm Krauss Webb, London, England, assignor to International Standard Electric Corporation,

New York, N. Y.

No Drawing. Application January 11, 1936, Se-

rial No. 58,783. 1935 '13 Claims.

This invention relates to improvements in electrical insulating materials and to the manufacture thereof and has for its main object to provide an improved process of treating fibrous materials such as paper, tape, fabric, silk, artificial silk, yarn, string or the like with styrene to render such materials suitable for a variety of uses in electrical insulation and particularly inthe manufacture of condensers and cables and m the jointing of cables.

I. The present invention provides as a new article of manufacture an electrical insulating material comprising a base of fibrous material such as indicated above, impregnated with liquid styrene which is polymerised.

According to the present invention there is provided a process of manufacturing electrical insulating material comprising a base of fibrous material for example paper which is characterised in this that the fibrous base is impregnated with styrene or styrene mixture so as to fill the pores and interstices thereof and the styrene is subsequently converted into polystyrene without loss by volatilisation, To this end the polymerisation may be effected in a bath of liquid styrene or styrene mixture. Preferably the styrene mixture comprises styrene and a 'plasticiser such as diphenyl or its derivatives.

An important feature of the invention consists in treating the surface of the impregnated material after polymerisation for the removal of surplus polystyrene so as to restore the fibrous texture of the surface. Suitable materials for such treatment are solvents of polystyrene such as liquid styrene, chlorinated diphenyl or benzene.

In the manufacture of'such insulating material in roll form a difficulty is experienced in that the polymerisation of the styrene impregnating the roll of material results in a solid polymerised mass which cannot be unwound, but according to a feature of the present invention this difilculty is overcome by a process of treatment which includes impregnating the material with a mixture of styrene and a plasticiser (e. g. styrene 60% diphenyl or its derivatives 40%) polymerising the mixture in a closed vessel, unwinding the material whilst the polymerised mixture is'soft, either at the temperature of polymerisation, or at a raised temperature, subjecting the material to treatment for the removal of surplus styrene to restore the fibrous texture of the surface and calendering and rewinding the material, preferably with a. suitable lubricant whereby the material may be readily unwound in subsequent use.

In Great Britain March 8,

The nature and uses of the invention will be better understood from the, following description thereof in which it has to be assumed that the paper or fabric may first have been esterified for example partially acetylated and so rendered less hygroscopic. It must also be assumed that the term styrene refers to styrene or a mixture of styrene with a suitable plasticiser introduced to increase the flexibility.

The roll of paper or fabric is impregnated in any suitable manner as by the vacuum impregnation process, with a suitable mixture of styrene and plasticiser (e. g. styrene chlorinated diphenyl 40%) which is polymerised under excess of the mixture in a closed vessel. The roll on removal is a solid mass, but if the polymerisation has been carried out gradually at C. the plasticised styrene will become sufficiently soft and sticky at 120 C. to permit unwinding. When unwound the tape or sheet may be smoothed by passing through rolls (which may be heated). If required it may then be wound while still sticky direct into position for insulating purposes. If, however, it is required to be stored for work in the field or on a machine it should be cooled and treated to remove the surplus styrene and restore the fibrous nature of the material as by being passed through a bath of lubricant. Suitable lubricants are styrene (liquid) chlorinated diphenyl, benzene, or, in general, solvents of polystyrol. The choice of the lubricating liquid depends upon the purpose for which the tape or sheet is being prepared. After the tape or sheet has been passed through the lubricant it may be wound again in a. roll. It will then be in a state in which it may easily be unwound and rewound as required without further treatment. It may, in fact, be handled similarly to ordinary impregnated or unimpregnated paper or fabric.

Metal foil maybe attached to the paper or fabric after the styrene has been polymerised and while it is hot and sticky and before the material passesthrough the smoothing rolls.

The invention is however not restricted to the manufacture of insulation in roll form as it may be applied to insulating sheets or panels in which case plasticisers may not be required. The base material of paper, cardboard, fabric, wood, or the like would be impregnated with styrene and polymerised in a closed mould, and to effect removal after polymerisation the mould could be chilled whereby the contraction is suificient to break the contact between the product and. the mould.

Moreover it is quite practicable to polymerise at a temperature at which the resultant product is soft, and therefore, in a state for immediate unwinding or separation. as an example the styrene impregnated material could be polymerised at 135 C. and unwound or separated immediately at this temperature without further heating.

The softening temperature of pure styrene after polymerisation is of the order of ION C. This is a starting point and it would probably be necessary to reach 130 to 140 C. in order to get efilcient winding. The addition of plasticizers would reduce this temperature.

The primary uses of polymerlsed styrenated paper or fabric according to this invention are in connection with cable joints and terminations as described in my copending applications Ser. No.

745,931, filed September 28, 1934, and Ser. No. 35,960, filed August 13, 1935, the material replacing the paper impregnated with styrene which is thereafter polymerised in situ.

I new material.

-There are, however, as mentioned above certain additional and special applications of this The material may, for example, be employed in constructing joints of the ordinary oil-impregnated type or in insulating coils in oilfilled transformers or similar electrical apparatus. In such applications the material provides great er security against electrical breakdown, compared with the ordinary oil-irnpregnated tape or fabric used. It also provides a certain measure of imperviousness to the oil or compound used and may restrictthe movementfof such insulating fluid. This effect of imperviousness may be enhanced in a simple way. If the material is heated during or after its application to the apparatus it can be rendered sticky and adherent. If, therefore, it is applied with an overlap a wrapping of an impervious nature is obtained.

An application of this nature is the use of prepolymerised styrenated paper or fabric as an impervious covering to a lead covered cable to inhibit the passage of moisture or corrosive fluids and acting in a manner similar to wrappings of bitumenised paper or fabric or to wrappings of self-vulcanising rubberised materials.

Another application is the insulation of low tension cables in a manner similar to that achieved by applying varnished carnbric tapes.

In laminated paper insulation which is impregnated with a fluid (or with a fluid which is afterwards solidified either by cooling or by polymerisation) it is realised that the main dielectric weakness lies in the films of free impregnating medium existing between consecutive laminations of paper. It would be an advantage if such films could be relieved of part of the electrical stress imposed in service. The impregnated paper could easily withstand greater electrical stresses. It is known that stress could be transferred from the films to the laminations of impregnated paper ates the laminations and eventually the grading effect disappears. If, however, the laminations are made of polymerised styrenated paper or fabric of relatively low specific inductive capacity and dielectric loss, and if after winding or wrapping the apparatus is impregnated with a mixture containing components of high specific inductive capacity or dielectric loss then the solid laminations of low specific inductive capacity are-mesa and'loss are separated by thin fllmsof material of high specific inductive capacity and/or loss, and the overall resistance to electrical breakdown is greatly increased. This mixture may be a styrene mixture which is subsequently polymerised in situ. l

In the special case of condensers high specific inductive capacity throughout is of particular importance since given values of capacity can be obtained with greatly reduced dimensions. in this case the following procedure can be followed. The prepolymerised styrenated paper or fabric can be made of increased specific inductive ca-' pacity value by incorporation in the styrene mixture of components of high specific inductive capacity. The impregnating compound which may be polymerised in situ can be made of even higher specific inductive capacity so that the overall specific inductive capacity is increased and the desired grading of electrical stress is simultaneously obtained. I

A suitable material for admixture with the im-. pregnating compound to raise the specific inductive capacity is nitrobenzene. A suitable material for admixture to raise the dielectric loss is an alcohol such as B phenyl ethyl alcohol.

In forming polymerised styrenated paper or fabric the first step is evacuation and impregnation with unpolymerised liquid styrene. Since styrene has a high vapour pressure arrangements can be made whereby after the initial drying and evacuation the paper or fabric passes. on its way to the impregnating bath through an atmosphere of styrene vapour which is being continuously impregnation will occur and the pores of the fibres will be penetrated. Owing to the low viscosity of the liquid styrene impregnation will be very rapid even at low temperatures. The paper may therefore be very thoroughly impregnated by passing into the impregnating bath at a speed consistent with that of the initial capillary rise in the paper. The paper may be then wound in a pad or roll under styrene and subsequently polymerised.

The methods described above are applicable to the formation of polymerised styrenated yarn or string which may be used for .various purposes in connectionwith telephone cables and wires. but it is clear that the invention is not limited in its scope to the particulars given above. For example, the invention may be applied to textile insulated wire which may be impregnated with styrene and polymerised after the manner dethe like, of electrical insulating material which comprises impregnating tapes, sheets or the like, of fibrous material with styrene or styrene mixture so as to fill the pores and interstices thereof, subsequently converting the styrene into polystyrene at an elevated temperature while the fibrous material is immersed in a bath of liquid styrene or styrene mixture whereby loss of styrene by volatilization during polymerization is inhibited, thereafter removing the fibrous material from said bath and subsequently treating the fibrous material to remove any surplus of polystyrene.

2. A process of manufacturing tapes, sheets or the like, of electrical insulating'material which comprises impregnating tapes, sheets or the like,

- of fibrous material with styrene or styrene mixture by vapor impregnation in an atmosphere of styrene vapor so as to fill the pores and interstices thereof, thereafter passing said tapes, sheets or the like of fibrous material into a bath of liquid or styrene mixture, subsequently converting the styrene into polystyrene by the ap-' plication of heat while the fibrous material is immersed in a bath of liquid styrene or styrene mixture whereby loss of styrene by volatilization during polymerization is inhibited, and thereafter removing the fibrous material from said bath.

3. A process according to claim 1, in which the fibrous material is esterified so as to render it less hygroscopic before impregnation.

4. A process according to claim 1, in which after polymerization of the impregnated fibrous material so much of the polymerized styrene is removed from the surface of the fibrous material as to restore the fibrous texture of said surface,

thereafter said'material is calendered, and then styrene at a temperature of at least approximately 120 C.Wh1l the fibrous material is immersed in a bath of liquid styrene or styrene mixture whereby loss of styrene by volatilization during polymerization is inhibited, thereafter removing the fibrous material from said bath and subsequently treating the fibrous material toremove any surplus of polystyrene.

6. A process according to claim 1 wherein the fibrous base is impregnated with a mixture comprising approximately 60% styrene and approximately 40% chlorinated diphenyl.

7. A process according to claim 1 wherein the surface of the impregnated material after polymerization is treated with a solvent of polystyrene such as liquid styrene, chlorinated diphenyl or benzene for the removal of polystyrene to resto the fibrous texture of the surface.

components of high specific inductive capacity for example nitrobenzene, are incorporated 8. A process accordingto'elaim 1 inwhich the impregnated material is polymerized in roll form and the said roll is unwound while the polymerised mixture is soft either at the temperature of polymerisation or at a raised temperature, and after treatment for the removal of surplus styrene the material is rewound in a condition in which it can be readily unwound for subsequent use.

9. A process of manufacturing electrical insulating material which comprises impregnating a base of fibrous material with styrene or styrene mixture so as to fill the pores and interstices thereof, subsequently converting the styrene into polystyrene at an elevated temperature while the fibrous material is immersed in a bath of liquid styrene or styrene mixture whereby loss of styrene by volatilization during polymerization is inhibited, thereafter removing the fibrous material from said bath. subsequently treating the fibrous material to remove any surplus of polystyrene, and thereafter calendering the fibrous material.

10. A sheet, tape or the like, of electrical insulating material which comprises a sheet, tape or. the like, of esterified fibrous material intimately impregnated with polymerized styrene or styrene mixture which fills vthepores and interstices thereof, the said polymerized styrene or styrene mixture being integrally united in and among said fibrous material by polymerization in situ and the said material being free from voids resulting from loss of styrene by volatilization durins P lymerization.

11. A process according to claim 1, in which the impregnated material is rolled up and then polymerised in roll form beneath the surface of a bath of styrene and the said roll is subsequently unwound and subjected to a treatment for the removal of surplus polystyrene.

12. A process according to claim 1, in which metal foil is attached to the material preferably after the styrene has been polymerised and while it is hot and sticky, for, example by passing the material and metal foil between smoothing rollers.

13. A process according to claim 1, wherein 45 the impregnating mixture.

JOHN 

